default to success and hide indicators if not needed

packages/stream_chat/lib/src/models/attachment.dart

@@ -58,7 +58,7 @@ class Attachment {
   final AttachmentFile file;
 
   /// The current upload state of the attachment
-  final UploadState uploadState;
+  UploadState uploadState;
 
   /// Map of custom channel extraData
   @JsonKey(includeIfNull: false)
@@ -123,10 +123,15 @@ class Attachment {
     this.actions,
     this.extraData,
     this.file,
-    this.uploadState,
+    UploadState uploadState,
   })  : id = id ?? Uuid().v4(),
         title = title ?? file?.name,
-        localUri = file?.path != null ? Uri.parse(file.path) : null;
+        localUri = file?.path != null ? Uri.parse(file.path) : null {
+    this.uploadState = uploadState ??
+        ((assetUrl != null || imageUrl != null)
+            ? UploadState.success()
+            : UploadState.preparing());
+  }
 
   /// Create a new instance from a json
   factory Attachment.fromJson(Map<String, dynamic> json) {

packages/stream_chat_flutter/lib/src/attachment/attachment_upload_state_builder.dart

@@ -27,7 +27,9 @@ class AttachmentUploadStateBuilder extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    if (attachment.uploadState == null) return Offstage();
+    if (message.status == null || message.status == MessageSendingStatus.sent) {
+      return Offstage();
+    }
 
     final messageId = message.id;
     final attachmentId = attachment.id;

packages/stream_chat_flutter/lib/src/attachment/file_attachment.dart

@@ -227,6 +227,18 @@ class FileAttachment extends AttachmentWidget {
           },
         );
 
+    if (message.status == null || message.status == MessageSendingStatus.sent) {
+      trailingWidget = IconButton(
+        icon: StreamSvgIcon.cloudDownload(color: theme.colorTheme.black),
+        padding: const EdgeInsets.all(8),
+        visualDensity: VisualDensity.compact,
+        splashRadius: 16,
+        onPressed: () {
+          launchURL(context, attachment.assetUrl);
+        },
+      );
+    }
+
     return Material(
       type: MaterialType.transparency,
       child: trailingWidget,